1. Field of the Invention
The present invention relates to a light amount control apparatus for use with a copying machine or an image reader.
2. Description of the Related Art
As well known, an image forming apparatus, such as a copying machine, comes in three varieties, the apparatus with a manual exposure control function, the apparatus with an automatic exposure control function, and the apparatus with both the above functions. In the apparatus with the manual exposure control function, an exposure of an exposure lamp (an amount of light emitted from a light emitting component) is manually controlled. In the apparatus with the automatic exposure control function, the light amount is automatically controlled in accordance with an optical density on an original. Most of the exposure control system in the copying machine is based on the control that varies a lamp voltage applied to the lamp by an analog technology.
More specifically, for the exposure control, the copying machine with the manual exposure control function uses a variable resistor attendant with a manual operation by an operator or the combination of a voltage dividing resistor and a switching means. In the later case, the voltage divider provides a plurality of DC voltages, and the switching means such as a relay selects any of those DC voltages. A DC voltage that is thus set and corresponds to the lamp voltage applied to the exposure lamp is generated and applied to a lamp regulator (voltage stabilizing circuit), thereby providing a controlled exposure. In the copying machine with the automatic exposure control function, an amount of light reflected from an original is detected by a photo sensor such as a photo diode. An analog computing circuit subtracts the output voltage of the photo sensor from a reference voltage, thereby providing an exposure control voltage.
Recent market needs require additional functions for the copying machine, such as the zooming function of copy magnification (continuously changing of the magnification) and a twin color copying function (copying with two colors). This makes the analog control circuit complicated. Consequently, the number of controls to be operated by service men has been increased. An automatic exposure processing section is used for obtaining a fixed image density against an original of a certain density. The processor applies a linear approximation to the relationship of the lamp voltage and different magnifications for each of the differences between the magnifications and a reference magnification of 100%. An overall level adjusting section is also used for correcting the lamp voltage against variations in manually set copy conditions and sensitivity. The above two sections use digital attenuators. Variations in the exposure by the lamp and in the sensitivity of the photo sensor against a magnification as set are corrected by a central processing unit (CPU). When a color copy mode is set up or a magnification is changed, a peripheral speed of the drum changes, and an exposure also change. The exposure change must be corrected. The above processings and corrections increase the number of analog switches and controls for adjustments. Further, the circuitry becomes large in size and complicated, and hence the reliability of the copying machine incorporating such circuitry is degraded.
Therefore, in the exposure control circuit (light control apparatus), reduction of the circuit size and the improvement of reliability must urgently be attained. Reduction of the signal processing by the analog technology in the exposure control circuit would greatly improve reliability. The introduction of software technology using a CPU into the assembling and adjustment in the manufacturing stage would lead to improvement of the reliability, and reduction of labor.
The introduction of software technology into the exposure control has studied on the basis of the pre-estimation. Software controlled exposure control technique has almost completely developed, and many exposure control circuits have been marketed. A typical automatic exposure control circuit which is based on the combination of the digital technology and the analog technology is disclosed in U.S. Pat. No. 4,699,502. In this type of the control circuit, the technical development now progressing is to implement the entire exposure control into a complete digital circuit.
In the automatic exposure control, the lamp voltage applied to the exposure lamp must be varied in accordance with optical densities of originals, in real time. It is very difficult to technically realize this. For this reason, the exposure control system which allows the reduction of the analog control and is adaptable for the automatic exposure adjustment, has not yet been provided, so far as we know.